CN1940115B - High-strength, high-toughness weldable steel and method of manufacturing components therefrom - Google Patents

Abstract

Disclosed are a weldable steel of high strength and high toughness and a method of producing members of machine parts. The steel consists essentially of, by weight %, C: 0.10-0.16%, Si: 0.05-0.50%, Mn: 1.3-2.3%, Cu: up to 0.5%, Ni: up to 0.5%, Cr: up to 0.5%, Mo: up to 0.3% and Ti: 0.025-0.035%, and the balance of Fe and inevitable impurities, and satisfying the condition that the weld-cracking susceptibility, Pcm, defined by the formula 1A below is less than 0.35, and the condition that the manganese equivalent, Mneq, defined by the formula 2A below is larger than 2.0. 1A: Pcm=C(%)+Si(%)/30+Mn(%)/20+Ni(%)/60+Cr(%)/20+Mo(%)/15 +Cu(%)/20 2A: Mneq=Mn(%)+Cu(%)+Ni(%)/2+Cr(%)+Mo(%).

Description

High-strength, high-toughness weldable steel and method of manufacturing components therefrom

technical field

The invention relates to high-strength and high-toughness weldable steel. The invention also relates to methods of using such steels for the manufacture of steel components for components such as automotive components.

Background technique

In the case of manufacturing various machine parts from steel materials, if two or more parts can be welded to form the shape of the part, it will be easy to manufacture complex products. Thus, it is possible to reduce the number of parts by integrating two or more parts hitherto joined by screws and nuts into one part, resulting in a reduction in the weight of the parts and enabling a reduction in manufacturing cost. However, in the case of parts requiring high strength and high toughness, there is a problem that steel having these properties has poor weldability, and thus, it is difficult to manufacture desired parts by combining specific members. People are caught in a dilemma. In order to improve the weldability of steel, it is necessary to choose an alloy composition with lower carbon content, and low carbon steel has low hardness, low toughness and low strength.

In order for the steel to maintain high weldability, it must not reduce the toughness of the heat-affected parts around the welded parts. Usually, due to the martensitic transformation caused by the heat generated in welding and the subsequent rapid cooling process, the hardness of the heat-affected part will be very high to the level of 400HV, so that the heat-affected part becomes brittle and the weld cracking. Since the hardness of the steel after martensitic transformation mainly depends on the carbon content, in order to avoid excessive hardening of the heat-affected part, it is necessary to maintain the content of various components whose content increases to reduce the hardness, especially the content of carbon. From this point of view, an index "Index of Weld-Cracking Susceptibility (Index of Weld-Cracking Susceptibility)" is known and used to maintain the content of components whose content increases to reduce hardness, hereafter it will be abbreviated as "Pcm".

On the other hand, too low a carbon content makes the steel insufficient in strength. The countermeasure to this problem is to improve the hardenability of the steel by adjusting the content of other alloying elements while maintaining the carbon content, whereby the depth of the hardened layer becomes deeper and the average hardness of the welded product is high, thereby maintaining the strength of the product . From this point of view, an index "Manganese Equivalent (Manganese Equivalent)" (hereinafter abbreviated as "Mneq") for determining the minimum content of alloying elements affecting hardenability has been discussed.

As a low-yield ratio-high-strength steel for building construction or large-scale structures such as bridges, a steel having a specific alloy composition consisting of (volume %) 5-30% polygonal ferrite has been proposed , 3-15% MA (a mixture of martensite and austenite) and the balance of bainite, and the average size of MA is up to 5 microns, as a material with good toughness and weldability ( Japanese Patent Laid-Open No. 2004-315925). However, this patent document discloses only the results of thermal cycle tests simulating welding for weldability (HAZ-toughness).

Research conducted by the present inventors was to find a method of maintaining the toughness of heat-affected parts in the manufacture of steel parts in which the base metal maintains the necessary strength and toughness while satisfying the conditions for combining the above with weld cracking susceptibility and hardenability The relevant two indicators are chosen at appropriate values. They found useful steels with specific alloy compositions and found that applying specific process conditions to them could solve the above-mentioned problems.

Contents of the invention

The purpose of the present invention is to take advantage of the knowledge of the inventors and to provide a steel which has high strength and high toughness, and which is still weldable. It is also an object of the present invention to provide a method for manufacturing machine component components from such steel. The term "weldable (weldable)" here means not only that the steel can be welded without weld cracking, but also that the welded parts have the advantageous property of sufficiently high toughness.

The basic composition of the basic alloy composition of the weldable steel with high strength and high toughness according to the present invention is, by weight percentage: C: 0.10-0.16%, Si: 0.05-0.50%, Mn: 1.3-2.3%, Cu: 0.5% at most, Ni: 0.5% at most, Cr: 0.5% at most, Mo: 0.3% at most and Ti: 0.025-0.035%, the balance being Fe and unavoidable impurities, and satisfying the conditions: by the following The weld cracking susceptibility Pcm defined by Equation 1A is less than 0.35, and the manganese equivalent Mneq defined by Equation 2A below is greater than 2.0.

1A: Pcm=C(%)+Si(%)/30+Mn(%)/20+Ni(%)/60+Cr(%)/20+Mo(%)/15+Cu(%)/20

2A: Mneq.=Mn(%)+Cu(%)+Ni(%)/2+Cr(%)+Mo(%)

Description of drawings

FIG. 1 is a conceptual diagram of a process for manufacturing a steel member according to a conventional technique or the present invention.

Fig. 2 is a conceptual diagram of a process for manufacturing a steel member according to a preferred embodiment of the present invention.

Fig. 3 is a conceptual diagram of a process for manufacturing a steel member in a more preferred embodiment of the present invention.

Fig. 4 is a graph showing the relationship between forging temperature and Charpy impact value or Vickers hardness obtained in working examples of the present invention.

Detailed ways

The steel of the present invention may contain B: 0.0003-0.005% in addition to the above alloy components. Adding an appropriate amount of B can increase the hardenability of the steel and is generally preferred. In the case where B is contained in the alloy, the above formulas 1A and 2A become the following formulas 1B and 2B:

1B: Pcm=C(%)+Si(%)/30+Mn(%)/20+Ni(%)/60+Cr(%)/20+Mo(%)/15+Cu(%)/20 +5B(%)

2B: Mneq.=Mn(%)+Cu(%)+Ni(%)/2+Cr(%)+Mo(%)+0.5

The method for manufacturing a steel member according to the present invention uses the above-mentioned steel composed of an alloy containing or not containing B and includes one of the following processing and heat treatment steps:

1) Forging at a temperature of 1050 ° C or higher to obtain the shape of the component, after cooling, reheating to A ₃ transformation temperature or higher temperature, quenching and hardening, and tempering to a certain hardness (the implementation in Figure 1 plan)

2) Forging at a temperature of 1050 ° C or higher to obtain the shape of the component, and directly quenching and hardening after forging, and tempering to a certain hardness;

3) Forging at a temperature higher than 1050°C but not exceeding 1150°C to obtain the shape of the component, and directly quenching and hardening after forging, and tempering to a certain hardness (the embodiment in Figure 2);

4) First forging at a temperature higher than 1050°C, and then at least one further forging to obtain the shape of the component, wherein the last forging is carried out at a temperature of 900-1000°C, and directly quenched and hardened after the last forging, and tempered to determine hardness; and

5) Forging at a temperature higher than 1050°C but not exceeding 1150°C, at least one further forging to obtain the shape of the component, wherein the last forging is carried out at a temperature of 900-1000°C, and directly quenched and hardened after forging, and tempered to Determined hardness (embodiment in Figure 3).

The forging step for obtaining components from steel material is usually carried out at relatively high temperatures, for example around 1250° C., which are prone to deformation. The forging method used in the present invention, which may be called semi-hot forging, is carried out at relatively low temperatures, such as above the _A3 transformation point but below 1100°C, which together with a suitable selection of weld cracking susceptibility and manganese equivalents, Capable of producing high strength and high toughness, which have been difficult to reconcile with each other.

The relatively low forging temperatures discussed above increase toughness by refining the martensitic structure after hardening. In order to take advantage of this mechanism, it is preferable to choose the lowest possible temperature allowed by the forging equipment, in the range of 900°C or higher, but not exceeding 1000°C. Then, as seen from the data of the working examples below, higher toughness can be achieved in welded parts, whereby superior parts can be produced.

The forging operation can be performed in two or more steps. At this time, the preferred final forging step is carried out at the lower temperature mentioned above to obtain better results, followed by direct quench hardening. This will give the same effect as if the whole forging was done at low temperature. Such a sequence of steps is chosen to combine an early forging with large deformation at a relatively high temperature where deformation is easy, and a later or remaining forging at a relatively low temperature. Forging performed at temperatures in the range of 900-1000° C. may be so-called hot coining with small deformation.

The reason why the alloy composition of the steel here is determined as above is explained below.

C: 0.10-0.16%

Carbon is an essential component to ensure the strength of the matrix. A small content below 0.10% does not give the desired strength. On the other hand, too much added affects weldability and results in lower toughness in heat-affected portions. Therefore, the upper limit is set at 0.16%.

Si: 0.05-0.50%

Silicon acts as a deoxidizer in steel. For effective use, 0.05% or more of Si is added. Too much addition will reduce the weldability and toughness of the steel, therefore, the addition amount must be at most 0.50%.

Mn: 1.3-2.3%

Manganese is also a deoxidizer. In the steels here, manganese is the first member of the components in the manganese equivalent formula. In order to obtain the necessary manganese equivalent and ensure strength, 1.3% or more of manganese is added. On the other hand, too much manganese increases the susceptibility to weld cracking causing weld cracking and further, reduces the toughness of welded parts. Thus, the added amount of manganese is at most 2.3%.

Cu: up to 0.5%

Copper appears in the manganese equivalent formula. Adding an appropriate amount of Cu improves the hardenability and contributes to the strength of the steel. A large amount of addition affects the toughness of steel, so the upper limit of addition is 0.5%.

Ni: up to 0.5%

Nickel contributes to the hardenability of steel, but has little effect on the sensitivity of welding cracking, so an appropriate amount of Ni should be added. Since this is an expensive material, the upper limit is set at 0.5% from an economical point of view.

Cr: up to 0.8%

Chromium is also an element that appears in the manganese equivalent formula and increases hardenability. Too much content affects the susceptibility to weld cracking, so the addition must be at most 0.8%.

Mo: up to 0.3%

Molybdenum and nickel contribute to hardenability as well as chromium. Because this metal is expensive, it is recommended to add it in small amounts up to 0.3%.

Ti: up to 0.06%

Titanium combines with nitrogen to form TiN, which contributes to increased strength. In order to ensure this effect, a certain amount of Ti is added. However, if the added amount is too large, the toughness of the portion affected by heat will be low. The upper limit of addition is 0.06%. The preferred range is 0.015-0.05%.

B: If added, 0.0003-0.005%

Before quenching, boron segregates on austenite grain boundaries and inhibits ferrite transformation to improve hardenability. Therefore, it is recommended to add a certain amount of boron. However, boron addition is not necessary if the manganese equivalent is as high as 2.0 or more to give sufficient hardenability. In the case of addition, a suitable amount is within the range of 0.0003-0.005%.

Since the welding cracking sensitivity of the steel member obtained by the method of the invention is suppressed very low, none of the welded parts has a hardness as high as 400HV, so the problem of cracking can be avoided during the welding process, and the welded parts have high toughness. Moreover, this steel with high hardenability and sufficient hardness is achieved throughout the component by quenching after forging. As a result, parts made by welding these members have high strength.

Example

Steels having alloy compositions shown in Table 1 (% by weight, balance Fe) were prepared. These steels are heated to 1100°C and forged, with a height reduction of 50% to make a block material with a thickness of 30mm. These materials were hardened and 3mm thick test pieces were removed from the hardened material and tempered at 465°C for 1 hour.

Two test pieces of each steel were welded by lap fillet welds. The filler is the same material as the base metal. Hardness testing is carried out on such fillet welded components. The results are shown in Table 2. Weldability is measured by the maximum hardness of the base metal, and those with a hardness below 400 HV are recorded as "good". Hardness testing is performed in the middle of the base metal in the thickness direction. Those of 250 HV or higher were evaluated as "good", and those below 250 HV were rated as "bad". Also given in Table 2 is the reason why the comparative examples are outside the claims of the present invention.

Steels "A", "B" and "C" which are working examples of the present invention satisfy both the weldability and hardness requirements of the base metal.

Comparative Examples "D" through "H" performed poorly in one or both of base metal weldability and hardness due to the following reasons:

D: Weldability is low because the carbon content is too large, and the Pcm value is outside the scope of the present invention;

E: The hardness of the base metal is too high, and Mneq is outside the required range due to insufficient Mn content;

F: The hardness of the base metal is low. Because the steel does not contain boron, Mneq is outside the scope of the present invention;

G: Solderability is low. Although alloying elements are within the scope of the invention, Pcm is outside the scope; and

H: The hardness of the base metal is too high. Although the content of alloying elements is in range, Mneq is out of range.

Table 1

Table 2

Then, the working example steel "A" and the comparative example steel "E" were forged with an area reduction of 65%, and then quenched and tempered according to the following four working and heat treatment procedures.

1) Hot forging/reheating-quenching/tempering (traditional technology, embodiment in Figure 1)

Comparative Example Steel "E" was hot forged at 1200°C, reheated to 900°C and quenched → tempered at 465°C for 1 hour.

2) hot forging/reheating-quenching/tempering (the embodiment of the present invention, the embodiment in Fig. 1)

Working example steel "A" was hot forged at 1200°C, reheated to 900°C and quenched → tempered at 465°C for 1 hour.

3) low temperature forging-quenching/tempering (according to the preferred embodiment of the present invention, the embodiment of Fig. 2 and Fig. 3)

Working example steel "A" was forged at a controlled temperature of 1100°C → tempered at 465°C for 1 hour;

Working example steel "A" was forged and quenched at a temperature controlled at 1100°C → forged at 900-1000°C by coining and quenched → tempered at 465°C for 1 hour;

4) low temperature forging/quick cooling/tempering (comparative examples outside the scope of the present invention)

Working example steel "A" was forged and quenched at a temperature controlled at 1100°C → forged at 800°C and quenched by coining → tempered at 465°C for 1 hour.

The above-mentioned forged and heat-treated products were subjected to a Charpy impact test to determine the impact value at 23°C, and a hardness test to determine the Vickers hardness. The relationship between forging temperature and impact value or Vickers hardness is shown in Fig. 4 . Fig. 4 shows that the known material has insufficient hardenability, so that the hardness (strength) after heat treatment is low, whereas the steel of the present invention has sufficient hardenability and exhibits satisfactory hardness and toughness. Also, in the steel of the present invention, when the final forging temperature is low, the strength and toughness can be more improved due to finer grains. However, if the final forging temperature is too low, the processing is carried out in the low temperature austenite region, therefore, it will accelerate the ferrite transformation or pearlite transformation, and cause the hardenability to decrease. At this time, the transformation of martensite is insufficient, and the hardness (strength) will drop significantly.

Claims (8)

1. use the method for steel as made steel member, it comprises step: under 1050 ℃ of perhaps higher temperature, forge this steel and obtain the member shape, reheat is to A ₃Transition point or higher temperature, chilling, and be tempered to the hardness that is lower than 400HV,

The alloy composition that said steel has does, by weight percentage: C:0.10-0.16%, Si:0.05-0.50%; Mn:1.3-2.3%, Cu: be at most 0.5%, Ni: be at most 0.5%; Cr: be at most 0.8%, Mo: be at most 0.3% and Ti: maximum 0.06%, and surplus is Fe and unavoidable impurities; And satisfy condition: by the following defined weld cracking susceptibility of formula 1A Pcm less than 0.35, and by the following defined manganese Equivalent of formula 2A Mneq greater than 2.0

1A：Pcm＝C(％)+Si(％)/30+Mn(％)/20+Ni(％)/60+Cr(％)/20+Mo(％)/15+Cu(％)/20

2A：Mneq＝Mn(％)+Cu(％)+Ni(％)/2+Cr(％)+Mo(％)。

2. according to the method for claim 1; Wherein except the alloy compositions that limits in the claim 1; This steel also contains B:0.0003-0.005%; And satisfy condition: by the following defined weld cracking susceptibility of formula 1B Pcm less than 0.35, and by the following defined manganese Equivalent of formula 2B Mneq greater than 2.0:

1B：Pcm＝C(％)+Si(％)/30+Mn(％)/20+Ni(％)/60+Cr(％)/20+Mo(％)/15+Cu(％)/20+5B(％)

2B：Mneq＝Mn(％)+Cu(％)+Ni(％)/2+Cr(％)+Mo(％)+0.5。

3. use the method for steel as made steel member, it comprises step: under 1050 ℃ of perhaps higher temperature, forge this steel and obtain the member shape, and direct chilling after forging, chilling also is tempered to the hardness that is lower than 400HV,

The alloy composition that said steel has does, by weight percentage: C:0.10-0.16%, Si:0.05-0.50%; Mn:1.3-2.3%, Cu: be at most 0.5%, Ni: be at most 0.5%; Cr: be at most 0.8%, Mo: be at most 0.3% and Ti: maximum 0.06%, and surplus is Fe and unavoidable impurities; And satisfy condition: by the following defined weld cracking susceptibility of formula 1A Pcm less than 0.35, and by the following defined manganese Equivalent of formula 2A Mneq greater than 2.0

1A：Pcm＝C(％)+Si(％)/30+Mn(％)/20+Ni(％)/60+Cr(％)/20+Mo(％)/15+Cu(％)/20

2A：Mneq＝Mn(％)+Cu(％)+Ni(％)/2+Cr(％)+Mo(％)。

4. according to the method for claim 3; Wherein except the alloy compositions that limits in the claim 3; This steel also contains B:0.0003-0.005%; And satisfy condition: by the following defined weld cracking susceptibility of formula 1B Pcm less than 0.35, and by the following defined manganese Equivalent of formula 2B Mneq greater than 2.0:

1B：Pcm＝C(％)+Si(％)/30+Mn(％)/20+Ni(％)/60+Cr(％)/20+Mo(％)/15+Cu(％)/20+5B(％)

2B：Mneq＝Mn(％)+Cu(％)+Ni(％)/2+Cr(％)+Mo(％)+0.5。

5. claim 3 or 4 method are wherein forged selected temperature at 1050 ℃ or higher, but are no more than 1150 ℃ scope.

6. use the method for steel as made steel member; It comprises step: at first this steel is forged being higher than 1050 ℃ or higher temperature; Be then at least once other forging to obtain the member shape, the last forging carried out 900-1000 ℃ temperature, and forges directly chilling of back the last time; Chilling also is tempered to the hardness that is lower than 400HV

The alloy composition that said steel has does, by weight percentage: C:0.10-0.16%, Si:0.05-0.50%; Mn:1.3-2.3%, Cu: be at most 0.5%, Ni: be at most 0.5%; Cr: be at most 0.8%, Mo: be at most 0.3% and Ti: maximum 0.06%, and surplus is Fe and unavoidable impurities; And satisfy condition: by the following defined weld cracking susceptibility of formula 1A Pcm less than 0.35, and by the following defined manganese Equivalent of formula 2A Mneq greater than 2.0

1A：Pcm＝C(％)+Si(％)/30+Mn(％)/20+Ni(％)/60+Cr(％)/20+Mo(％)/15+Cu(％)/20

2A：Mneq＝Mn(％)+Cu(％)+Ni(％)/2+Cr(％)+Mo(％)。

7. according to the method for claim 6; Wherein except the alloy compositions that limits in the claim 6; This steel also contains B:0.0003-0.005%; And satisfy condition: by the following defined weld cracking susceptibility of formula 1B Pcm less than 0.35, and by the following defined manganese Equivalent of formula 2B Mneq greater than 2.0:

1B：Pcm＝C(％)+Si(％)/30+Mn(％)/20+Ni(％)/60+Cr(％)/20+Mo(％)/15+Cu(％)/20+5B(％)

2B：Mneq＝Mn(％)+Cu(％)+Ni(％)/2+Cr(％)+Mo(％)+0.5。

8. the method in the claim 6 or 7 is wherein forged selected temperature first at 1050 ℃ or higher, but is no more than 1150 ℃ scope.

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